The invention relates to a process for the electrostatic charging of recording materials which pass underneath a spray corona, the recording materials being driven and supported by means of rollers, and an apparatus for implementation of the process.
Electrophotographic recording materials, such as printing plates, are usually charged by the plate being laid onto a flat table and sucked up against the table surface by means of a vacuum device and a charging device, such as, for example, a spray corona, and subsequently being moved over the printing plate at a constant distance from the table.
European Patent Application No. 0 100 643 describes an apparatus for the recording and electrophotographic development of partial images of a microfilm which includes three processing stations alongside one another. The first station charges the film and exposes the first partial image with a copy, the second station develops the electrostatic charge image, and the third station dries and fixes the toner image. During the individual process steps, the film to be exposed and developed lies in each case on a flat base and is not moved during the individual process step. The charging station consists of a single corona spray wire, on either side of which semi-cylindrical corona electrodes are arranged to effect a uniform configuration or distribution of the charges emanating from the spray corona wire.
A charging device of an electrophotographic unit is known from European preliminary published specification No. 0 095 217, in which the corona wire has the shape of a loop and runs between two rollers. The corona wire rotates at a speed greater than the speed of the photoconductor to be charged. The ends of the corona wire are tied together to form the loop and the tie is embedded in plastic, which surrounds it in the form of a bead. With this known charging apparatus there occurs a charging of the photoconductor, which is passed over the charging apparatus, which the loop of corona wire runs around. Special precautions are not taken to keep the distance between the photoconductor and the corona wire constant over the entire length of the charging apparatus.
U.S. Pat. No. 4,374,616 relates, inter alia, to a corona charging apparatus which can be moved back and forth over a recording material to be charged.
Apparatuses of this type operate satisfactorily and a good uniformity is achieved of the charge potential on the surface of the recording material to be charged, for example, a printing plate. Such corona charging apparatuses which can be moved back and forth are expensive and take up a great deal of space in a unit for the preparation of printing plates since they require guide rails and a reversible motor. The corona charging apparatus must be equipped with an advance and return, as a result of which the time for the necessary return is lost in the sequence for the preparation of the printing plate.
For reasons of time and cost savings, recently the charge-principle has been altered such that the recording material or the printing plate to be charged is passed underneath a stationary corona charging apparatus with the aid of transport rollers. In this arrangement, difficulties may occur to the extent that the constant distance required for a uniform charging between the printing plate and the corona charging apparatus with one or two wires depends on the dimensional and positional constancy (flatness) of the printing plate, the plate rigidity and the support possibilities underneath the corona charging apparatus. In particular, distance discrepancies due to undulations and insufficient support of the printing plates lead to unequal charge potentials on the printing plate. In particular, errors of this type occur to a very great extent whenever the printing plates have edge undulations. Dimensional and positional discrepancies of up to about 2 mm with respect to the evenness of the printing plates keep recurring on account of edge undulations, even in the case of good plate production, and can only be avoided with great effort and a correspondingly high reject level of defective plates. Even if the printing plates are guided over flat supporting surfaces or tables, an uneven charge distribution on account of the edge undulations of the printing plates cannot be avoided.